High capacity lightwave transmission systems attempt to optimize bandwidth usage by employing wavelength division multiplexing. This is accomplished by using optical sources such as lasers, each designed to operate at a different wavelength (frequency) from the other lasers.
Recently, a number of designs have been proposed for tunable semiconductor lasers exhibiting a large number of available optical wavelengths. One such tunable laser is shown in U.S. Pat. No. 5,373,517 copending U.S. patent application Ser. No. 08/019,952 filed Feb. 19, 1992). This laser is fabricated with optical amplifiers, optical waveguides and an N.times.N waveguide grating router which are all integrated on a semiconductor wafer and disposed within a resonant optical cavity formed between two reflective end surfaces.
Waveguides formed in the semiconductor wafer carry light between the reflective end surfaces and the frequency router. The waveguides terminate on the input and output ports of the router at locations determined solely by first order diffraction within the free-space region of the router.
An addressable optical amplifier is formed in a portion of each input and output waveguide for the router. Each amplifier can be selectively addressed via a bias current to provide optical gain or optical loss. Amplification results within a particular waveguide when the amplifier is activated by a bias current. If the amplification provides sufficient optical gain to overcome intra-cavity losses, lasing is sustained at the wavelength determined by the path through the router. Moreover, selected amplifiers provide sufficient loss within their respective waveguides to prevent transmission of optical energy when no bias current or a low bias current is supplied to the amplifier.
In operation, this tunable laser is set to operate at a predetermined wavelength when a single pair of amplifiers is selected to operate in their separate amplification modes. This pair of amplifiers determines a particular path through the frequency router and, therefore, the wavelength of the light output. Light at the desired wavelength is extracted directly from the spot at which the particular path terminates on the reflective end surface. The laser is rapidly tunable to any of N wavelengths by selecting an appropriate pair of amplifiers.
While this laser has shown improved performance with respect to tuning speed, tuning range, and wavelength selectivity, its overall length is sufficiently long to prohibit high speed operation using direct modulation or an intra-cavity modulator. Modulation is limited to several hundred Mbps.